The present invention relates to methods of making biocompatible foams having a micropatterned surface disposed on and integral with at least one surface of the foam.
Open-cell, porous, biocompatible foams have been recognized to have significant potential for use in the repair and regeneration of tissue. Early efforts in tissue repair focused on the use of amorphous biocompatible foam as porous plugs to fill voids in bone. Brekke, et al. (U.S. Pat. No. 4,186,448) described the use of porous mesh plugs composed of polyhydroxy acid polymers such as polylactide for healing bone voids. Several attempts have been made in the recent past to make tissue engineering (TE) scaffolds using different methods. For example, U.S. Pat. No. 5,522,895 (Mikos) and 5,514,378 (Mikos, et al.) using leachables; U.S. Pat. No. 5,755,792 (Brekke) and 5,133,755 (Brekke) using vacuum foaming techniques; U.S. Pat. No. 5,716,413 (Walter, et al.) and 5,607,474 (Athanasiou, et al.) using precipitated polymer gel masses; U.S. Pat. No. 5,686,091 (Leong, et al.) and 5,677,355 (Shalaby, et al.) using polymer melts with fugitive compounds that sublimate at temperatures greater than room temperature; and U.S. Pat. No. 5,770,193 (Vacanti, et al.), 5,769,899 (Schwartz, et al.) and 5,711,960 (Shikinami) using textile-based fibrous scaffolds. Hinsch et al. (EPA 274,898) describes a porous, open-cell foam of polyhydroxy acids with pore sizes from about 10 to about 200 xcexcm for the in-growth of blood vessels and cells. The foam described by Hinsch could also be reinforced with fibers, yarns, braids, knitted fabrics, scrims and the like.
The above techniques have limitations in producing porous scaffolds with controlled surface textures. The scaffolds are smooth-walled and lack the surface features that encourage the attachment of cells, their proliferation, and their differentiation into phenotypes appropriate for the specific tissue type.
Surface features are known to influence cell adhesion, migration, proliferation, and differentiation. The adhesion and migration of osteoblasts along surface features on implants has been studied extensively by many groups, including Jack Ricci et al. (See Morphological Characteristics of Tendon Cells Cultured On Synthetic Fibers, J. Biomed. Mater. Res., J. Ricci et al., Vol. 18, pages 1073-87, 1984.) The influence of surface topography on the proliferation and differentiation of osteoblast-like MG-63 cells has been described in Surface Roughness Modulates the Local Production of Growth Factors and Cytokines by Osteoblast-like MG-63 Cells, J. Biomed. Mater. Res., Kieswetter et al., Vol. 32, pages 55-63, 1996 and Effect of Titanium Surface Roughness on Proliferation, Differentiation, and Protein Synthesis of Human Osteoblast-like Cells (MG63), J. Biomed. Mater. Res., Martin et al., Vol. 29, pages 389-401, 1995.
The ideal implant surface is one that promotes tissue ingrowth and healing. In the case of bone, four surface properties of implants play a role in the attraction of primitive mesenchymal stem cells, and their differentiation into osteoblasts at the bone implant interface during the healing process. The four surface properties are composition, surface energy, topography and roughness. This is described in further in Underlying Mechanisms at the Bone-biomaterial Interface, J. Cell. Biochem., Z. Schwartz et al., Vol. 56, pages 340-7 1994. Another study described in An Evaluation of Variables Influencing Implant Fixation by Direct Bone Apposition, J. Biomed. Mater. Res., K. Thomas et al., Vol. 19, pages 875-901, 1985, has shown that texture, defined as a combination of roughness and topography, plays a greater role in cell response than implant material.
Textured implant surfaces have been shown to produce better bone fixation than smooth surfaced implants, see Removal Torques For Polished and Rough Titanium Implants, Int. J. Maxillofac. Impl., L. Carlsson et al., Vol. 3, pages 21-24, 1988 and The Influence of Various Titanium Surfaces on the Interface Shear Strength Between Implants and Bone, Clinical Implant Materials: Advances in Biomaterials, H.-J. Wilke et al., Vol. 9, pages 309-314, 1990. Surface patterning has been shown to have a great effect on cell behavior, both in tissue culture experiments (See Optimization of Surface Micromorphology for Enhanced Osteoblast Responses In Vitro, Int. J. Oral Maxillofac. Impl., K. Bowers et al., Vol. 7, pages 302-310, 1992) and in vivo (See Influence of Surface Characteristics on Bone Integration of Titanium Implants: A Histomorphometric Study in Miniature Pigs, J. Biomed. Mater. Res., D. Buser et al., Vol. 25, pages 889-902, 1991.) In addition, the effect of implant surface texture on the type of extracellular matrix (ECM) produced has been described in Orientation of ECM Protein Deposition, Fibroblast Cytoskeleton, and Attachment Complex Components on Silicon Microgrooved Surfaces, J. Biomed. Mater. Res., E. Den Braber et al., Vol. 40, page 291, 1998.
Several multi-step processes for making textured surfaces on polymeric foams are known. Shalaby and Roweton (U.S. Pat. Nos. 5,969,020, 5,899,804, 5,847,012, 5,677,355 and WO 9505083) describe a process for creating patterns of pores on foam surfaces via the extraction of a fugitive material to foam the polymer. Another approach to make foams with patterned surfaces is proposed by Griffith et al. (WO9947922).
The limitation of the above-described techniques for forming foams with patterned surfaces is that they require multiple processing steps. As the number of processing steps is increased, the possibility of rejection of the final product is increased, as well as the cost of the resulting product.
Yet another approach to making foams with patterned surfaces is described by Vacanti et al. (WO9640002). Here, several solid free-form fabrication (SFF) processes are described. Examples of SFF methods include stereo lithography (SLA), selective laser sintering (SLS), ballistic particle manufacturing (BPM), fusion deposition modeling (FDM), and three-dimensional printing (3DP). An additional approach to create porous polymeric structures with channels is described in A Polymer Foam Conduit Seeded with Schwann Cells Promotes Guided Peripheral Nerve Regeneration, Tissue Engineering, Hadlock et al., Vol. 6, pages 119-127, 2000.
All processes noted above either require an additional step to produce the textured surface via etching, micromachining, punching, leaching, round razor or laser drilling, or other similar process, or are very complex and require specialized expensive equipment. Further, the resulting patterned surfaces are not completely integrated into the structures and may be subjected to differences in degradation and tissue response under in vivo conditions.
The present invention provides a simple, one-step process for making biocompatible foams containing a micropattern disposed on and integral with at least one surface of the foam. The structure of the micropattern provides organization at the microstructure level and a template that facilitates cellular invasion, proliferation and differentiation, thus ultimately resulting in regeneration of functional tissue.
Summary of Invention
The present invention is directed to a method of making a foam, the method comprising contacting a polymer solution with a surface of a mold, the solution comprising dissolved therein a biocompatible polymer, the mold comprising disposed on at least one surface thereof a three-dimensional negative configuration of a predetermined micropattern to be disposed on and integral with at least one surface of the foam, lyophilizing the solution while in contact with the micropatterned surface of the mold, thereby providing a lyophilized, micropatterned foam, and removing the lyophilized, micropatterned foam from the mold. Foams prepared according to the invention comprise a predetermined and designed micropattern on at least one surface, which pattern is effective to facilitate tissue repair, ingrowth or regeneration, or is effective to provide delivery of a protein or a therapeutic agent.
The tissue response to scaffolds with such designed surfaces can be tailored depending on the desired response. Changing the surface micropattern will alter the bioabsorption profile, and will provide a different microenvironment for cell adhesion and migration, both of which are advantageous in a variety of medical applications.
Preferred micropatterned structures are particularly useful for the regeneration of tissue between two or more different types of tissues. For a multi-cellular system in the simplest case, a first cell type could be present on one side of the foam scaffold with a specific, predetermined surface micropattern designed to facilitate growth of the cell, while a second cell type could be present on the other side of the foam scaffold with a different predetermined micro-pattern designed to facilitate growth of the second cell type. Examples of such regeneration include, without limitation, (a) vascular tissue: with smooth muscle on the outside and endothelial cells on the inside to regenerate vascular structures; and (b) osteochondral tissue: by implanting with a surface micropattern that attracts chondrocytes on one surface of the foam and a different micro-structure that attracts osteoblasts or pre-osteoblasts on the opposing surface.